The parallel mass spectrometer (PMS) is defined as a mass spectrometer (MS) which consists of two or more sets of ion extraction means, mass resolution devices (quadrupole mass filters, magnetic sections, etc.) and ion detectors (electron multiplier, etc.) connected in parallel. It permits two or more ion beams characteristic of the introduced sample to be extracted, mass resolved, and detected simultaneously. The PMS differs from the commonly known tandem mass spectrometer in which two or more mass resolution devices are connected in tandem and in which only one beam is formed and only one detector is normally employed. The PMS acts virtually in the same manner as two independent mass spectrometers, positioned side by side, but with considerable saving in the construction cost and with more efficient utilization of the sample materials and the obtained data.
The PMS is known to exist in the prior art in various forms. Svec and Flesch (J. Mass Spect. Ion Phys., 1, 41, 1968) first reportedly constructed a PMS consisting of two magnetic sectors for simultaneously analyzing positive and negative ions extracted from an electron impact ionization (EI) ion source. Another PMS was reported (D. Henneberg, U. Henrichs and G. Schomburg, J. Chromatography, 122, 343, 1975) in which a quadrupole mass analyzer with its own ion source and detector was added to a gas chromatograph (GC)--magnetic sector mass spectrometer system. In U.S. Pat. No. 4,266, 127, a new type of PMS consisting of two quadrupole mass analyzers for simultaneous positive ion negative ion chemical ionization (SPINICI) operation was described.
One obvious advantage of the PMS over the conventional single mass spectrometer is that in the PMS the two detected sample ion beams can be of different charge polarities and/or derived from different ion formation mechanisms such as EI, CI, photoionization, neutral or charged particle bombardment, etc. The information obtained from these two ion beams are often complementary to each other and greatly facilitate the analysis of the sample in question.
When interfaced with a sample separation device, such as gas chromatograph (GC) or liquid chromatograph (LC), the PMS can also provide a significant advantage over the conventional single mass spectrometers. For example, in a PMS the first mass resolution device can be operated in a select ion monitoring (SIM) mode. This is achieved by tuning the first mass resolution device to one ion (single ion monitoring) or several ions (multiple ion monitoring) of interest. The obtained SIM trace (SIM chromatogram or mass chromatogram) is far superior over the reconstructed mass chromatogram trace (described below) in terms of the chromatographic resolution and detection sensitivity (by a factor of 100 or more). In the meantime, the second mass resolution device can be operated in a repetitive mass scanning mode which yields mass spectra for the qualitative identification of each chromatographic peak.
Alternatively, the first mass resolution device can be operated in a total ion current (TIC) detection mode. This is normally achieved through rapid mass scanning with signal integration. The obtained TIC trace (TIC chromatogram) is also much better than the reconstructed TIC trace (Described below) in terms of the chromatographic resolution and retention index accuracy. Furthermore, if the first mass resolution device is a quadrupole type which is capable of operation in a RF-only mode, the resultant TIC trace is of even better chromatographic resolution and detection sensitivity (by a factor of 100 or more) than any other type of TIC trace.
In conventional single mass spectrometers, various forms of ion beam monitors are also known to exist which allow a fraction of the extracted ion beam to be detected independent of the operation of the mass resolution device. However, these types of ion beam monitors have been found to be unacceptable for quantitative GC-MS analysis, because the obtained ion signal generally consists of a high level noise, resulting from species, such as the GC carrier gas, CI reagent gas, GC column bleed, and instrument background, commonly presented in a GC-MS environment. For this reason, these ion beam monitors are rarely used in a GC-MC experiment, except for tuning the mass spectrometer. On the contrary, the TIC trace obtained with mass resolution device filtration, such as in a PMS system is largely free from such noise, because the mass resolution device can be properly programmed to screen out most unwanted noise.
However, in spite of the above-described advantages, the PMS as operated in the SIM (or TIC)--repetitive mass spectra acquisition mode offers little improvement over the conventional single mass spectrometer in obtaining the mass spectra data.
The repetitive mass spectra acquisition operation is widely adopted in most modern GC-MS-computer systems. However, there are several fundamental problems associated with the repetitive mass spectra acquisition operation which are generally understood but can not be avoided because of lack of suitable solutions.
One of the major reasons for employing the repetitive mass spectra acquisition operation is that during a GC-MS experiment one does not know when a GC component will enter the mass spectrometer and needs to be analyzed by obtaining a mass spectrum data. The only solution available then is to perform the repetitive mass scan and blindly acquire and process all mass scan data at rapid rates during the entire GC run, so that all GC components flowing from the GC column and entering the mass spectrometer will be analyzed, regardless of the appearance time of the GC components. However, since only the mass spectra recorded in coincidence with the appearance of the GC peaks are of value, all of the other spectra recorded are simply a waste of computer time and memory storage, except for a few spectra which are utilized for background subtraction.
In a typical repetitive mass spectra acquisition operation, the mass scanning rate is adjusted so that there are at least ten complete mass scans across each GC peak, in order to minimize the spectrum distortion, resulting from the rapid change of the sample concentration in the ion source, and to ensure that a good mass spectrum can be recorded at the GC peak top region. For a GC peak width of 10 seconds the minimum acceptable mass scanning rate will be one mass scan cycle per second. This means that during an one-hour period of GC run, as many as 3600 (1.times.60.times.60) mass spectra have to be acquired, processed and stored in a computer system. An extremely large computer system is therefore required. This is especially true for a high resolution GC--high resolution MS system. The demand on the interfaced computer capability is therefore enormous and rarely possible.
One common treatment of the acquired repetitive mass scan data is that of obtaining a reconstructed TIC trace by summing all ion currents (in digital form) within each scan and plotting this sum vs. the scan number. Alternatively, one or more reconstructed mass chromatograph traces can be obtained by plotting the intensity of one or more ions of particular m/e values in each scan data vs. the scan number. These reconstructed TIC or mass chromatographs traces generally resemble a normal gas chromatograph trace recorded continuously with a GC detector, such as a flame ionization detector or a mass spectrometer operated in the real TIC or SIM mode. However, there is one significant difference in that there is only one date point for each mass scan in the reconstructed TIC or mass chromatograph trace. The achievable chromatographic resolution in this trace largely depends on the number of mass scan cycles which can be performed within each GC peak retention time. In the above example, for a GC peak width of 10 seconds and a mass scan rate at one mass scan cycle per second, a total of only 10 data points can be recorded across each GC peak as represented by the reconstructed TIC or mass chromatograph trace. The poor chromatographic resolution resulting from this limited number of data points severely compromised the GC performance and makes it very difficult to obtain an accurate peak retention index, which is the most important parameter for peak identification in gas chromatography.
It is clear from the above discussion that in order to prevent excessive deterioration in the chromatographic resolution, the mass scan rate for a repetitive mass spectra acquisition GC-MS analysis should be adjusted as rapid as practical. Unfortunately, a rapid mass scan rate brings with it the problem of deterioration of the mass spectrum detection sensitivity and spectra quality. Both these two factors, to a large extent, depend on the duration of each mass scan period, in which the longer the scan period, the better the detection sensitivity and spectra quality. The contradicting nature of the GC resolution and MS spectra quality and sensitivity requirements makes it practically impossible to achieve a GC-MS analysis without compromise in the performance of either GC or MS or both in a repetitive mass spectra acquisition operation.
The compromise in GC and/or MS performance becomes even more severe in the case of a high resolution GC-high resolution MS system operated under repetitive mass spectra acquisition mode. Because of the restraint on the MS spectra quality requirement, the maximum achievable high resolution mass scan cycle is only in the order of 5-10 seconds. This time period is far inadequate for most high resolution (capillary) columns which normally yield peak width of 10-20 seconds. The chromatographic resolution of the reconstructed TIC or mass chromatograph trace is of a quality too poor to be of significant analytical value. To circumvent this problem, most modern high resolution mass spectrometers are simply operated under low mass resolution when interfaced with a high resolution GC column. This is indeed a severe loss to a mass spectrometer originally designed for high mass resolution and which cost several times that of a low resolution mass spectrometer.
With the advance of GC separation technology, the average GC peak width becomes narrower and narrower. It is not uncommon that in some glass capillary GC columns the resolved sample component has a peak width of only 1-2 seconds. This type of high performance GC columns is generally considered incompatible with a conventional GC-MS system operated under a repetitive mass spectra acquisition mode. In order to achieve minimum acceptable chromatographic resolution for this narrow peak, the repetitive mass scan rate must be set at 5-10 mass scan cycles per second, which is practically impossible, even for most low resolution mass spectrometers having a maximum useful scan rate of only 1-2 mass scan cycles per second. As a result, GC columns of lower quality must be used. These usually require longer operation time and thus suffer from lower sample analysis throughput.
As mentioned previously, in a GC-MS analysis, the mass spectra recorded during the repetitive mass scan operation are generally distorted as a result of the rapid change of the sample concentration within the ion source of the mass spectrometer. This problem is generally ignored, because there is no simple way to correct for this distortion. Obviously, this problem makes it difficult to compare the obtained spectra with reference spectra for the unknown compound identification.
It is not unusual that in the GC-MS analysis there may be a complex sample which may consist of hundreds of components but in which only a few of the components are of interest for analysis. Specific analysis of these few components is not possible with a repetitive mass spectra acquisition operation. Such an operation is a non-discriminatory analysis method, which will faithfully acquire, process and store the spectra of all components, regardless of their analysis needs. The large volume of unwanted spectra not only wastes the precious computer time and memory storage, but also complicates the final spectra analysis.